1. Field of the Invention
The present invention relates generally to an apparatus and method for outputting an image, and more particularly, to an apparatus and method for compensating the black level of an image signal, prior to transmission to an external image output device.
2. Description of the Related Art
When a captured image of an object is converted into an image signal, color coordinates such as Red, Green, Blue (RGB), Cyan, Magenta, Yellow (CMY), Hue Saturation, Intensity (HSI), YUV, and YCbCr are generally used to represent standard colors.
The RGB color system represents an image using three primary colors of light, that is, red, green and blue signals. An image signal at RGB color coordinates is typically used for an image output device such as a Cathode Ray Tube (CRT) or a Liquid Crystal Display (LCD).
While RGB color coordinates have information about primary color signals, YUV color coordinates have information about chrominance signals. Y denotes luminance, U equals B−Y, and V equals R−Y (i.e. U=B−Y, V=R−Y).
YCbCr color coordinates are a digital representation of YUV color coordinates and are usually employed to represent Standard Definition (SD) or High Definition (HD) Television (TV) images or to compress Joint Photographic Experts Group (JPEG) images. In YCbCr, Cb corresponds to U of YUV and Cr corresponds to V. The YCbCr standard is defined in International Telecommunication Union (ITU) R BT. 601(SD) and ITU R BT. 709(HD). The YUV color coordinates including the YCbCr color coordinates are based on the principle that the eyes are most sensitive to light intensity. The YUV color coordinates advantageously decrease the total data amount of an image signal by appropriately maintaining a data amount allocated to the Y component and reducing a data amount allocated to the U or V component to which the eyes are insensitive.
Camcorders are generally tuned to two available signal ranges. Specifically, one type of camcorder creates images for a TV, a monitor, or broadcasting in compliance with a standard, whereas the other type of camcorder is tuned without complying with a standard, considering a Personal Computer (PC)-using environment, so that a high-quality image may be output through a PC.
The former type of camcorder complies mainly with ITU R BT.601(SD) and ITU R BT.709(HD). According these standards, the signal range of an image signal is expressed in 8 bits, ranging from 16 to 235. The upper bound may be set to exceed 235.
The image signal range of the latter type of camcorder is an 8-bit full range from 0 to 255.
A created image may be output to an external image output device, that is, a display in two manners.
One such manner is to directly output an image to, for example, a TV or a monitor. An 8-bit Y component of 16 in a YUV signal is mapped to 0 in the RGB color system. That is, YUV color coordinates are converted into RGB color coordinates by Equation (1) (ITU R BT.601) and Equation (2) (ITU R BT.709), as follows.
                              [                                                                      Y                  601                  ′                                                                                                      C                  B                                                                                                      C                  R                                                              ]                =                              [                                                            16                                                                              128                                                                              128                                                      ]                    +                                    [                                                                    65.481                                                        128.553                                                        24.966                                                                                                              -                      37.797                                                                                                  -                      74.203                                                                            112                                                                                        112                                                                              -                      93.786                                                                                                  -                      18.214                                                                                  ]                        ×                          [                                                                                          R                      ′                                                                                                                                  G                      ′                                                                                                                                  B                      ′                                                                                  ]                                                          (        1        )                                          [                                                                      Y                  709                  ′                                                                                                      C                  B                                                                                                      C                  R                                                              ]                =                              [                                                            16                                                                              128                                                                              128                                                      ]                    +                                    [                                                                    46.559                                                        156.629                                                        15.812                                                                                                              -                      25.664                                                                                                  -                      86.336                                                                            112                                                                                        112                                                                              -                      101.730                                                                                                  -                      10.270                                                                                  ]                        ×                          [                                                                                          R                      ′                                                                                                                                  G                      ′                                                                                                                                  B                      ′                                                                                  ]                                                          (        2        )            
The other manner is to map an 8-bit YUV signal to an RGB signal without changing a luminance of 16, for use in a display of a PC.
FIGS. 1A to 2B compare an image captured by a camcorder (FIGS. 1A and 1B) complying with a standard and displayed on a TV (FIG. 1A) and a PC (FIG. 1B) with an image captured by a camcorder (FIGS. 2A and 2B) that does not comply with a standard and displayed on a TV (FIG. 2A) and a PC (FIG. 2B), in terms of image quality, according to the prior art.
Referring to FIGS. 1A and 1B, when an image captured by the camcorder complying with a standard is directly output to a TV or a monitor (FIG. 1A), the black level of the image is maintained as normal without quality degradation. However, if the image is output to a PC (FIG. 1B), the black level is higher than it actually is and thus a screen is brighter than normal. In addition, contrast decreases, thereby degrading video quality. The black level is the brightness level of the darkest (black) part of an image.
Referring to FIGS. 2A and 2B, when an image captured by the camcorder not complying with a standard is output to a PC (FIG. 2B), the black level of the image is maintained as normal without quality degradation. However, if the image is output directly to a TV or a monitor (FIG. 2A), clipping occurs to a dark part of the image, thereby make it very difficult to identify an image.